1. Technical Field
The present invention relates generally to digital communications protocols and in particular to biphase encoded signals that encode data utilizing signal transitions within bit cells. More particularly, the present invention relates to a device and method for detecting transition-encoded signals in which the transition within each bit cell occurs at the approximate midpoint of the bit cell. Still more particularly, the present invention relates to an improved Manchester code detector and method for implementing the same that processes both halves of a bit cell signal to determine the value of the received transition-encoded bit cell.
2. Description of the Related Art
Biphase codes are widely utilized for digital data transmission including pulse coded modulation (PCM) transmission. A common form of biphase code, known as Manchester code, has been specified for the IEEE 802.3 standard for baseband coaxial cable and twisted-pair Carrier Sense Multiple Access/Collision Detection (CSMA/CD) bus LANs. Manchester encoding has also been used in air interface applications such as Wireless Local Access Network (WLAN) specified by the IEEE 802.11 and 802.15.4 standards.
Manchester encoding, named for the University of Manchester, where its first recorded use occurred in the late 1940's, is widely utilized to provide clock and data information simultaneously via a single connection. Manchester encoding is a synchronous digital encoding technique utilized by the Open System Interconnection (OSI) physical layer to encode the clock and data of a synchronous bit stream. A description of the characteristics and nature defining Manchester codes, alternatively referred to as biphase codes, is presented in Stallings, Data and Computer Communications, 3rd Ed., pp. 90-92, the substance of which is incorporated herein by reference.
Manchester encoding is fundamentally distinguished from other digital encoding techniques in that the actual logic values embodying binary data to be transmitted over the cable or air-interface are not delivered as a sequence of signal levels, but are instead encoded as mid-bit signal transitions. Several types of such mid-bit signal transitions are possible depending on the modulation technique employed for transmission. For example, if the bit stream is amplitude modulated, the direction of a mid-bit transition between a first half-bit interval having a first amplitude and the subsequent second half-bit interval having a second amplitude determines the value of the bit cell. If the bit stream is frequency modulated, the bit cell logic value is determined by a mid-bit shift in frequency, and similarly for phase modulation.
Its transition-based nature enables Manchester encoding to overcome some of the limitations of level-based encoding, such as non-return to zero (NRZ) encoding, in which the value of each bit is determined by the signal value (amplitude or frequency, for example) over the bit period. Specifically, Manchester codes are “self-clocking” as the receiver detecting an incoming Manchester signal is able to detect and therefore utilize the mid-bit transition for reliable clock synchronization using a digital phase lock loop (DPLL) thus dispensing with the need for an additional clock input. This reliable clock recovery feature makes Manchester encoding well-suited for use with single-core transmission media such as optical fiber and coaxial cable. Additionally, the biphase nature of Manchester codes results in a balanced DC level across the signal.
Manchester codes are widely utilized in Ethernet and other networking architectures employing CSMA/CD. In order to detect the logic value of a Manchester encoded unit bit cell, conventional detectors/receivers sample and process a selected half of the bit cell, referred to herein as either the first or the second half-bit interval of the bit cell. The processed half-bit value can then be utilized to deduce the direction of the mid-bit transition as between two amplitude levels, frequency values, etc. A problem encountered with this signal detection technique is that only half, typically the second half, of each unit bit cell is processed for detection, resulting in half the received signal energy being wasted in terms of signal detection. Such half-bit detection results in a substantial reduction in the signal energy that is actually processed by the detector compared with the signal energy processed for a level-based signal detection in which the entire bit cell energy is processed during detection. The reduced received/processed signal energy results in an increased signal detection error rate and possibly the need for increased transmitter power.
It can therefore be appreciated that a need exists for an improved Manchester detector that utilizes the received Manchester signal energy more efficiently, resulting in a lower required transmission power and/or lower detector end bit error rate. The present invention addresses such a need.